TTurbine:

I experianced a major loss of power , due to the high boost pressure .... especially in the summer....

 

This is the problem, the DIN hp rating is at 20DegC so the stock engine "system" can keep the IAT below the critical timing retard threshold of 55degC but any increase in ambient temperature will trip  the IAT beyond that 55DegC and your power is going bye bye Smiley
 

The bigger VTGs are helpful in keeping the IATs down but in the GT2 RS Tuning rate the maximum with standard intercoolers about the same as Ruf @ 635PS and that is at DIN 20DegC......


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2009 997 GT2 659PS/827NM DIN